KrF excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. A typical prior art KrF excimer laser is depicted in FIG. 1 and FIG. 2. A pulse power module 2 provides electrical pulses lasting about 100 ns to electrodes 6 located in a discharge chamber 8. The electrodes are about 28 inches long and are spaced apart about 3/5 inch. Typical lithography lasers operate at a high pulse rate of about 1,000 Hz. For this reason it is necessary to circulate a laser gas (a typical example being about 0.1 percent fluorine, 1.3 percent krypton and the rest neon which functions as a buffer gas) through the space between the electrodes. This is done with tangential blower 10 located in the laser discharge chamber. The laser gasses are cooled with a heat exchanger also located in the chamber. Commercial excimer laser systems are typically comprised of several modules which may be replaced quickly without disturbing the rest of the system. Principal modules are shown in FIG. 1 and include:
Laser Chamber 8, PA1 Pulse Power Module 2, PA1 Output coupler 16, PA1 Line Narrowing Module 18, PA1 Wavemeter 20, PA1 Computer Control Unit 22, and PA1 Peripheral Support Sub systems.
The discharge chamber is operated at a pressure of about three atmospheres. These prior art lasers typically operate in a pulse mode at about 600 Hz to about 1,000 Hz, the energy per pulse being about 10 mJ and the duration of the laser pulses is about 15 ns. Thus, the average power of the laser beam is about 6 to 10 Watts and the average power of the pulses is in the range of about 700 KW.
At wavelengths below 300 nm there is no available technique for providing refractive systems with chromatic correction. Therefore, stepper lenses will have no chromatic correction capability. The KrF excimer laser operates at a nominal wavelength of about 248 nm and has a natural bandwidth of approximately 300 pm (full width half maximum, or FWHM). For a refractive system (with a numerical aperture &gt;0.5)--either a stepper or a scanner--the wavelength of the light source needs to be held substantially constant with variations and spread minimized to the picometer range. Current prior art commercially available laser systems can provide KrF laser beams at a nominal wave length of about 248 nm with a bandwidth of about 0.8 pm (0.0008 nm). Wavelength stability on the best commercial lasers is about 0.25 pm. With these parameters stepper makers can provide stepper equipment to provide integrated circuit resolutions of about 0.3 microns.
To improve resolution a more narrow bandwidth is required. For example, a reduction of a bandwidth to below 0.6 pm (FWHM) would permit improvement of the resolution to below 0.25 microns. As indicated above, the bandwidth is usually specified as the pulse width measured (on a chart of pulse power versus wavelength) as the full width at one half the maximum power of the pulse. Another important measure of the pulse quality is referred to as the "95% integral." This is the spectral width of the portion of the pulse containing 95% of the pulse energy. The desired 95% bandwidth is less than about 1.5 pm to 2.0 pm. However, the prior art KrF laser can only provide "95% integral" values of 3 pm over the lifetime of the laser.
It is known to use etalons in excimer lasers for wavelength control, either alone or in combination with gratings and/or prisms. The main disadvantage of etalons in a high power laser is that heat produced by the laser beam can distort the etalon changing the optical parameters of the etalon. This problem has been recognized and dealt with in at least two patents: U.S. Pat. No. 5,150,370 issued on Sep. 22, 1992 to Furuya et al. and U.S. Pat. No. 5,559,816 issued on Sep. 24, 1996 to Basting and Kleinchmidt. Both of these arrange for a laser as two polarization-coupled cavities with the main light generation taking place in the first cavity and the wavelength control being done by the etalon placed in relatively low-power second cavity. This technique, even though improving optical performance of the etalon, still is not able to reduce "95% integral" to required 1.5-2.0 pm; and also, this technique is quite complicated.
The performance of stepper equipment depends critically on maintaining minimum bandwidth of the laser throughout the operational lifetime of the equipment. Therefore, a need exist for a reliable, production quality excimer laser system, capable of long-term factory operation and having wavelength stability of less than 0.2 pm and a FWHM bandwidth of less than 0.5 pm and a 95% integral bandwidth of less than 2.0 pm.